The present invention relates to a gas diffusive electrode that can suitably be used for producing a fuel cell and a method of producing such an electrode. The present invention also relates to an electrochemical device.
The need for clean energy sources that can replace fossil fuel including petroleum has been strongly recognized in recent years. Particularly, hydrogen (gas) fuel has been attracting attention.
Hydrogen can operate as an ideal energy source that is clean and inexhaustible because it provides a large amount of chemical energy per unit mass and does not produce harmful substances nor gas that can warm the earth when it is burnt.
Recently, fuel cells adapted to turn hydrogen energy into electric energy have been developed vigorously. Expected applications of such fuel cells include large scale power generation plants, on-site non-utility power generation facilities and power sources of electric automobiles.
Such a fuel cell is typically designed to provide electromotive force by arranging a hydrogen electrode and an oxygen electrode with a film of a proton conducting body sandwiched between them and causing a cell reaction to take place by supplying fuel (hydrogen) and oxygen to the respective electrodes. The cell is normally produced by molding a film of proton conducting body, a fuel electrode (e.g., hydrogen electrode) and an oxygen electrode separately and then putting and binding them together.
Both the fuel electrode (e.g., hydrogen electrode) and the oxygen electrode (gas diffusive electrode) are mainly made of electro-conductive carbon grain and additionally comprise a catalyst layer carrying a catalyst metal such as platinum.
Conventionally, such a gas diffusive electrode is realized in the form of a sheet that is prepared by using electro-conductive powder or grain of carbon and catalyst grain carrying platinum as catalyst along with water-repellent resin such as fluorine resin and an ion conducting material or by directly applying catalyst grain, water-repellent resin and an ion conducting material onto a carbon sheet.
More specifically, a gas diffusive electrode of a solid polymer fuel cell is produced typically by applying particles carrying platinum onto a carbon sheet along with water-repellent resin and an ion conducting material in a manner as disclosed in Japanese Patent Application Laid-Open Publication No. 5-36418.
A gas diffusive electrode as used herein refers to an electrode having continuous pores that can diffuse working gas and showing electron conductivity.
When such a gas diffusive electrode is used as the hydrogen decomposing electrode of a fuel cell such as a solid polymer type fuel cell, the supplied fuel is ionized by the catalyst such as platinum and electrons generated as a result of ionization flow through the electro-conductive carbon powder or grain, while protons (H+) generated as a result of ionization of hydrogen flow to the film of an ion (proton) conducting body by way of the ion conducting material. It will be appreciated that, for the above process, gaps for allowing gas to flow, electro-conductive grain or powder of carbon, an ion conducting material for allowing ions to flow and a catalyst for ionizing fuel and an oxidizing agent need to be provided.
Thus, fuel is ionized by the catalyst such as platinum and electrons generated as a result of ionization are made to flow through the electro-conductive grain or powder of carbon while the ionized hydrogen (protons) are made to flow to the ion conductive film by way of the ion conducting material. For the above process, gaps for allowing gas to flow, electro-conductive grain or powder of carbon, an ion conducting material for allowing ions to flow and a catalyst for ionizing fuel and an oxidizing agent need to be provided.
If the fuel is hydrogen, a reaction of2H2→4H+4e−
takes place in the gas diffusive electrode (catalyst layer) of the fuel cell, whereas a reaction ofO2+4H+4e−→2H2O
takes place in the oxygen electrode of the cell to produce water.
Meanwhile, in a fuel cell of the type under consideration, it is very important how to make the two reactions proceed smoothly in the respective electrodes for the purpose of improving the performance of the cell.
Particularly, the above described oxygen electrode is accompanied by problems including that the electrode reaction takes place only insufficiently to reduce the output level of the fuel cell because water (H2O) produced in the course of the reaction in the electrode adheres to the peripheries of the particles of the carbon powder or grain within the catalyst layer (gas diffusion layer) and reduces the gaps.